1. Field of the Invention
The present invention relates to a light emitting device including an element (hereinafter referred to as an EL element) in which a thin film (hereinafter referred to as an organic EL film) made of an organic compound capable of obtaining EL (Electro Luminescence) is interposed between electrodes.
2. Description of the Related Art
In recent years, a technique for forming a TFT (thin film transistor) on a substrate has been greatly developed, and its application and development to an active matrix type display device (light emitting device) has been advanced. Especially, since a TFT using a polysilicon film has higher field-effect mobility (also called mobility) than that of a conventional TFT using an amorphous silicon film, a high speed operation is achieved. Thus, it becomes possible to control a pixel, which has been conventionally performed by a driving circuit external to a substrate, by a driving circuit formed on the same substrate as the pixel.
In the active matrix type light emitting device like this, various circuits and elements are formed on the same substrate, so that various merits, such as reduction in manufacturing costs, miniaturization of an electro-optical device, improvement of yield, and reduction in throughput, can be obtained.
Moreover, research of an active matrix type light emitting device including an EL element as a self-luminous element has been made active.
The EL element has such a structure that an EL layer is interposed between a pair of electrodes (anode and cathode), and in general, the EL layer has a laminate structure. Typically, there is cited a laminate structure xe2x80x9chole transporting layer/light emitting layer/electron transporting layerxe2x80x9d proposed by Tang et al. of Kodak Eastman Company. This structure has very high luminous efficiency, and most of what has been studied at present adopts this structure.
In addition, the structure of layers laminated in the order of a hole injecting layer/hole transporting layer/light emitting layer/electron transporting layer or a hole injecting layer/hole transporting layer/light emitting layer/electron transporting layer/electron injecting layer on an anode may be adopted. The light emitting layer may be doped with a fluorescent pigment or the like.
In the present specification, every layer provided between a cathode and an anode is generically called an EL layer. Thus, all of the foregoing hole injecting layer, hole transporting layer, light emitting layer, electron transporting layer, electron injecting layer and the like are included in the EL layer.
A predetermined voltage is applied to the EL layer of the above structure from a pair of electrodes, and then recombination of carriers is produced in the light emitting layer to emit light. Note that in the present specification, a light emitting element formed of an anode, an EL layer and a cathode is called an EL element.
Since the deterioration of the EL layer included in the EL element is accelerated by heat, light, moisture, oxygen and the like, in general, the EL element is formed after wiring lines and TFTs are formed in a pixel portion in the manufacture of an active matrix type light emitting device.
Various methods are proposed as forming (film formation) methods of the EL layer. For example, a vacuum evaporation method, a sputtering method, a spin coating method, a roll coating method, a cast method, an LB method, an ion plating method, a dipping method, an ink jet method, a printing method and the like can be cited.
Further, in order to realize colorization of the light emitting device, for example, as disclosed in Japanese Patent Application Laid-open No. Hei. 10-012377, there is proposed a technique in which an ink jet method is used to form EL layers with different luminous colors for respective pixels.
In order to realize a high definition display in the light emitting device, an active matrix type is desirable. However, in the case where a plurality of EL layers are separately painted and are formed to realize colorization, there is a case where poor formation of the EL layer occurs due to a metal mask shift by the minute structure.
In order to solve the problem, in the present invention, a plurality of pixels arranged in one row in a pixel portion is called a pixel row, a plurality of pixels arranged in one column in the pixel portion is called a pixel column, and several kinds of EL layers are formed for every pixel group, such as a pixel row, a pixel column or a plurality of adjacent pixels, to realize multi-colorization in an active matrix type light emitting device.
In the present invention, among the pixel rows in the pixel portion, a first kind of EL layer is formed for a plurality of pixel rows from an end. Then, after a pixel row adjacent to the first kind EL layer is skipped, a second kind of EL layer is formed for a plurality of pixel rows adjacent to this. Note that a pixel row provided between the first kind of EL layer and the second kind of EL layer is referred to as a preliminary region in the present specification, and an EL layer is not formed in this preliminary region.
The preliminary region is provided as a margin for a case where a mask shift or the like occurs when the first kind of EL layer and the second kind of EL layer are formed.
Note that since a signal is not inputted to a pixel row existing in a preliminary region, even if the first kind of EL layer or the second kind of EL layer is formed in the preliminary region, there does not arise a problem.
Further, after the second kind of El layer is formed, a preliminary region is again provided in an adjacent pixel row. That is, the EL layer and the preliminary region are alternately provided like this to prevent poor quality from being produced at the time of formation of the EL layer in the pixel portion.
Note that it is desirable to provide one to five rows of the preliminary regions formed here, that is, it is desirable to provide two to six kinds of EL layers through the preliminary regions.
As luminescent materials forming the EL layer, although well-known materials can be used, in order to improve external quantum efficiency, it is necessary to use an organic compound (hereinafter referred to as a triplet compound), which can convert triplet excitation energy into light emission (phosphorescence), for at least one kind of EL layer. Incidentally, since a material used for normal light emission is a compound which can convert singlet excitation energy into light emission, the material is referred to as a singlet compound.
As the triplet compound, organic compounds disclosed in the following papers can be cited as typical materials.
(1) T. Tsutsui. C. Adachi, S. Saito, Photochemical Processes in Organized Molecular Systems, ed. K. Honda, (Elsevier Sci. Pub., Tokyo, 1991) p. 437.
(2) M. A. Baldo, D. F. O""Brien, Y. You, A. Shoustikov, S. Sibley, M. E. Thompson, S. R. Forrest, Nature 395 (1988) p. 151.
(3) M. A. Baldo, S. Lamansky, P. E. Burrrows, M. E. Thompso, S. R. Forrest, Appl. Phys. Lett., 75 (1999) p.4
(4) T. Tsutsui, M.-J. Yang, M. Yahiro, K. Nakamura, T. Watanabe, T. Tsuji, Y. Fukuda, T. Wakimoto, S. Mayaguchi, Jpn. Appl. Phys., 38 (12B) (1999) L1502.
The organic compounds disclosed in these papers are as follows:
[Chemical Formula 1]
The above molecular formula expresses a metal complex (hereinafter referred to as a platinum complex) having platinum of a third transition series element as center metal.
[Chemical Formula 2]
The above molecular formula expresses a metal complex (hereinafter referred to as an iridium complex) containing iridium as center metal.
Note that the triplet compound is not limited to these compounds, but a compound having the above structure and containing an element in group 8 to 10 of the periodic table as center metal can also be used.
The above triplet compound has higher luminous efficiency than the singlet compound, and the operation voltage (voltage required for making an EL element emit light) can be made low in order to obtain the same luminous brightness.
Further, in addition to the formation of the EL layer made of the same luminescent material for every plural pixel rows as described above, another form may be used which includes the EL layer formed by using the same luminescent material for a plurality of pixel columns or a plurality of adjacent pixels.
In addition, one kind of EL layer which is formed of the same luminescent material may occupy a region of 70-90% of the whole EL layers formed in the pixel portion.
By forming the EL layers as describe above, it is possible to form a multicolor light emitting device with high definition and enables reduction in consumed electric power.